1. Field of the Invention
The present invention relates to current control systems for an inverter which drives an inductive load such as an alternating-current motor, a linear motor or the like, by means of a pulse-width modulation (PWM) system and, more particularly, to a current control system for an inverter, which accurately follows a current command value even when a current actual value is at steady state with respect to the current command value and even when the current actual value is at transition state with respect to the current command value.
2. Description of the Related Art
Generally, an alternating-current motor 9 is driven by a voltage-type inverter 8 as shown in FIG. 1 of the attached drawings. A speed controller 1 computes and outputs current command values i.sub.q * and i.sub.d * of respective q-axis and d-axis on the basis of a differential value of a rotational angle .theta. detected by a rotational-angle sensor 13 which is mounted to the alternating-current motor 9, that is, a deviation between a rotational speed .omega. and a command value .omega.* of the rotational speed. A transformer 2 obtains current command values i.sub.u *, i.sub.v * and i.sub.w * of respective u-, v- and w-phases on the basis of the current command values i.sub.q * and i.sub.d *, and outputs the current command values to a current control unit 3. A pair of current detectors 10 and 11 detect phase current of the alternating-current motor 9. Further, an adder 12 obtains the remaining phase current. These various phase currents are fedback to the current control unit 3. At the current control unit 3, three comparators 4, 5 and 6 obtain deviations .DELTA.i.sub.u through .DELTA.i.sub.w between the current command values i.sub.u * through i.sub.w * and current detecting values i.sub.u through i.sub.w. A PWM controller 7 computes output potential commands for the respective phases on the basis of the deviations. The inverter 8 is PWM-controlled on the basis of the output potential commands for the respective phases, to control current of the alternating-current motor 9.
In such system, the current control unit 3 is an important component which influences or affects the control performance. Considered as the prior art are an instantaneous-value current control system, a mean-value current control system, a control system on the basis of a d-q axis, or the like.
FIG. 2 shows a hysteresis comparator system which represents the instantaneous-value current control system. In this hysteresis comparator system, a deviation .DELTA.i.sub.u between the current command value i.sub.u * and the current detecting value i.sub.u is obtained by a comparator 21. On the basis of the deviation .DELTA.i.sub.u, the respective phases of the inverter are controlled in an ON-OFF manner by a hysteresis comparator 22. That is, when the deviation .DELTA.i.sub.u increases above a positive threshold, an output from the hysteresis comparator 22 is brought to a HIGH level. On the other hand, when the deviation .DELTA.i.sub.u decreases below the threshold value of the deviation .DELTA.i.sub.u, the output from the hysteresis comparator 22 is brought to a LOW level. Such treatment or processing is effected independently of the respective v-phase and w-phase. In case of this system, the kaleidoscopic current detecting value is monitored, and the ON-OFF control is done on the basis of the monitored detecting value. Accordingly, response ability is superior. However, there is such a problem that, in a low speed range, control accuracy is deteriorated so that it is impossible to apply the system to positioning control. Further, a switching frequency f.sub.s largely varies dependent upon the operating condition and, particularly, increases abruptly during low speed. For this reason, it is required that the switching frequency f.sub.s during the low speed is set to a value equal to or less than an upper-limit value which is permitted or allowed by the inverter. The switching frequency f.sub.s is restrained to a considerably low value as a whole. As a result, a reduction in the current control accuracy is led with respect also to a high speed range. Moreover, noises are also large, and tone also varies dependent upon the operational condition, so that a feeling of unpleasant is given to a third person. The cause of this can be described as follows. That is, since voltage capable of being outputted by the inverter is determined in combination at the time the u-phase, the v-phase and the w-phase select the HIGH level or the LOW level, the voltage is expressed in terms of voltage vectors of 2.sup.3 =8. If the output levels are expressed in order of the u-phase level, the v-phase level and the w-phase level with the HIGH level being 1 and with the LOW level being 0, there are obtained (000), (001) . . . (111). If the binary expression is replaced by a decimal expression so that the voltage vectors are expressed with the value as an affix, there are obtained V.sub.0 through V.sub.7 as shown in FIG. 1. Here, for example, it is assumed that a desirable output voltage V.sub.x is in such a phase relationship that the desirable output voltage V.sub.x is brought to the following equation as shown in FIG. 1:
______________________________________ voltage of the v-phase &gt;voltage of the u-phase &gt;voltage of the w-phase ______________________________________
Then, since the direction of V.sub.x is approximate to the directions of V.sub.2 and V.sub.6, it is possible to efficiently control the desirable output voltage V.sub.x, if V.sub.2 and V.sub.7 corresponding respectively to the zero vectors and V.sub.6 and V.sub.0 are alternately selected. In case of the hysteresis comparator system, however, each phase freely determines the output voltage. Accordingly, V.sub.0 through V.sub.7 are disorderly selected, and a time mean value of V.sub.0 through V.sub.7 is controlled so as to be brought to V.sub.x. In this manner, the eight voltage vectors are selected non-uniformly, and the voltage vectors, that is, V.sub.1 and V.sub.5 in this case, totally opposite in direction to the desirable voltage V.sub.x are selected. This is the cause of a reduction in the control accuracy and an increase in the noise.
On the other hand, FIG. 3 shows a fundamental or basic circuit for the mean-value current control system. The circuit is constructed by the comparator 21, an amplifier 23 whose gain is K.sub.p, an amplifier 24 whose gain is K.sub.i, an integrator 25, an adder 26, a comparator 27, a polarity decider 28, and a triangular-wave generator comprising an integrator 29, and a hysteresis comparator 30. In this system, a voltage command value v.sub.u * is obtained as follows. That is, a value, in which an error .DELTA.i.sub.u between the current command value i.sub.u * and the current detecting value i.sub.u is multiplied by the proportional gain K.sub.p, is added to a value, in which the error .DELTA.i.sub.u is multiplied by the proportional gain K.sub.i and is integrated, thereby obtaining the voltage command value v.sub.u *, as represented by the following equation: EQU v.sub.u *=K.sub.p .multidot..DELTA.i.sub.u +K.sub.i .intg..DELTA.i.sub.u dt(1)
where .DELTA.i.sub.u =i.sub.u *-i.sub.u.
The voltage command value v.sub.u * is compared with a triangular wave e.sub.t by the comparator 27. A polarity of the comparison result is judged or decided by the code decider 28. Control is done such that, when v.sub.u * .gtoreq.e.sub.t, the output voltage is brought to the HIGH level, while, when v.sub.u * &lt;e.sub.t, the output voltage is brought to the LOW level. If such PWM control is effected for each of the u-phase, the v-phase and the w-phase, a timelike mean value of the output voltage from the inverter is into agreement with the voltage command values v.sub.u * through v.sub.w *. In this connection, the triangular wave e.sub.t is obtained such that the output from the integrator 29 is compared by the hysteresis comparator 30, and the compared value is inverted and fed-back to the integrator 29.
FIG. 3 illustrates only the u-phase. However, the same triangular wave e.sub.t is used to perform the similar control with respect also to each of the v-phase and the w-phase. For this reason, the voltage vector selected at that time is limited only to the voltage vector which is optimum at the point of time. In this connection, if the desirable output voltage V.sub.x is in a direction as shown in FIG. 1, v.sub.v *&gt;v.sub.u *&gt;v.sub.w *. From a magnitude relationship between v.sub.u * through v.sub.w * and the triangular wave e.sub.t, the voltage vector selected is limited only to V.sub.2 and V.sub.6 whose directions are the same as V.sub.x, and V.sub.0 and V.sub.7 which correspond to the zero vector. The voltage vector in the opposite direction is not selected. In order to cause the current detecting value i.sub.u to follow the current command value i.sub.u *, however, it is required that .DELTA.i.sub.u =0 in the above equation (1). Accordingly, the following relationship holds: EQU v.sub.u *=K.sub.1 .intg..DELTA.i.sub.u dt (2)
Thus, it will be required that the output from the integrator 25 follows the voltage to be applied to the motor, that is, the sum of a speed electromotive force at that time and a primary voltage drop. However, the speed electromotive force increases during the high-speed rotation, and a frequency of the speed electromotive force also increases. Accordingly, the integrator 25 cannot follow. As a result, the control accuracy is deteriorated. During the low speed, under a transient condition in which the current command value i.sub.u * varies largely, the output from the integrator 25 cannot respond in accordance with the variation in the current command value i.sub.u *. For this reason, the control accuracy is deteriorated.
As described above, the instantaneous-value current control system has problems in the low-speed range, and the mean-value current control system has problems in the high-speed range. These problems are caused by the following facts. First, the potentials of the respective three phases are controlled in spite of the fact that independent variables are two. That is, i.sub.u +i.sub.v +i.sub.w =0 holds and, accordingly, if the currents of the two phases are determined, other one phase is determined unconditionally. Since the three phases are controlled independently of each other, however, interference occurs between the phases. Thus, the control characteristic is deteriorated.
Secondly, in spite of the fact that the current to be controlled originally are the torque current and the exciting current, the phase current is in fact controlled for each phase. The phase current is expressed in terms of trigonometric function based on the torque current, the exciting current and the rotational angle. For this reason, it is impossible to separate the torque current and the exciting current from each other, so that interference occurs between the torque current and the exciting current. This makes it further difficult to improve the control characteristic.
To the contrary, a system for effecting control on the basis of a d-q axis has been considered, as disclosed in "Application to Brushless-Servomotor-Control for Variable Construction System", 1988, National Convention 135 of Electric Institute Industry Division. This method is as follows, as shown in FIG. 4. That is, in a transformation circuit 34, a torque current (q-axis current) i.sub.q and and exciting current (d-axis current) i.sub.d are obtained on the basis of the phase currents i.sub.u, i.sub.v and i.sub.w. A pair of adders 31 and 32 obtain an error .DELTA.i.sub.q between a torque-current command value i.sub.q * and the torque current i.sub.q, and an error .DELTA.i.sub.d between an exciting-current command value i.sub.d * and the exciting current i.sub.d. A voltage-vector selecting circuit 33 determines an output voltage of each phase on the basis of the torque-current error .DELTA.i.sub.q and the exciting current error .DELTA.i.sub.d. Here, the output voltage of each phase is determined on the basis of a map shown in FIG. 5. That is, the voltage vector is selected in accordance with the rotational angle of the magnetic flux at that time, on the basis of codes of the torque current error .DELTA.i.sub.q and the exciting current error .DELTA.i.sub.d and potentials are outputted correspondingly. At that time, however, the voltage vectors selected are only four types of voltage vectors opposite in direction to each other. These four types of voltage vectors are selected alternately to effect control such that the timelike mean value is brought to a desirable voltage. Originally, in order to obtain a superior control characteristic, two types of voltage vectors identical in direction with the desirable voltage and V.sub.0 and V.sub.7 corresponding to the zero vector must be selected. From this, it will be seen that this system is the PWM control which is much in current ripple and which is inefficient. Moreover, in case of this system, the PWM control is effected by selection of the voltage vectors. Accordingly, similarly to the instantaneous-value current control system and the mean-value current control system, it must be considered that the potentials of the three phases are controlled respectively, in spite of the fact that the independent variables are two. The voltage vectors are in combination of the potentials of the three phases, and selection of the voltage vectors are nothing but selection of the potentials of the three phases. Thus, the phases interfere with each other. As a result, the voltage vector in the opposite direction is selected so that the control is brought to the PWM control which is much in current ripple and which is inefficient. In addition, one voltage vector is selected on the basis of the codes of the torque current error .DELTA.i.sub.q and the exciting current error .DELTA.i.sub.d. Accordingly, at a stage in which the voltage vector is selected, control is effected under such a condition that the torque current and the exciting current are composed with each other. Thus, in the strict sense of the word, the torque current and the exciting current are not brought to non-interference. This also causes deterioration of the control characteristic which is much in current ripple.